How can I fix this about my duct tape wallets?

Okay, so recently I've got into making duct tape wallets, but can't seem to figure out the measurements. My wallets end up coming out uneven, and odd. I bought a ruler mat that is used for fabric, and even with that they come out wrong. It's almost as if my tape keeps going slanted, but I don't know if it's because my mat keeps getting slanted, I don't know. Does anyone have a good instructional because the ones I've found have stunk. Also does anyone have any tips or tricks for making a perfect wallet?